We encountered a problem on our email server and as an initial procedure I tried to read the logs, the log says that
relay=none, delay=141015, status=deferred (Name service error for yahoo.com: Host not found, try again)
so as an initial response i tried to check on /etc/resolve.conf, ping, dig and nslookup, but the problem is results are all ok, further looking for resources through google lead me to /var/spool/postfix/etc/resolve.conf so after changing my nameserver entries to a different DNS Server, Problems are now Ok.....